This invention relates to a valve of the proportional flow type operated by an electrical solenoid. More particularly, this invention relates to a valve having a high turn down ratio, i.e., one which can control flow rates ranging from very low, through intermediate, to very high magnitudes.
Proportional flow valves find utility in performing mixing and measurement functions. For example, proportional flow valves are used to accurately blend gasolines to achieve desired characteristics, such as particular octane ratings, to mix hot and cold water to obtain a desired temperature, and to dispense compressible and incompressible fluids, including liquids such as gasoline, and gases such as air and natural gas. Depending on the application for which a proportional flow valve is to be used, it may be necessary to maintain constant flow rates of a very low magnitude as well as constant flow rates of a very high magnitude, and constant flow rates of an intermediate magnitude between said high an low magnitudes.
In some prior art proportional valves, a main valve member is lifted off of and lowered onto a main valve seat to open and close the valve. The main valve member can be mounted at the center of a diaphragm. Such a valve is shown in U.S. Pat. No. 5,676,342. This valve permits a rate of fluid flow through the valve proportional to the amount of electric current flowing through the coil of the solenoid actuator controlling the valve. In this type of arrangement, the actuator behaves in a linear manner, i.e., the force produced by the solenoid armature is linearly proportional to the current applied to the solenoid. As a result, the solenoid armature works in a linear manner against a closing spring, which constantly urges the valve member toward the valve seat. In this way, the distance which the valve member is moved away from the valve seat is proportional to the amount of current applied to the solenoid.
Atop the main valve member is a pilot valve seat, which surrounds a pilot opening through the center of the main valve member. The plunger of a solenoid above the main valve member carries a pilot valve member which is lowered to seal the pilot valve opening in the main valve member and raised to open the pilot valve opening in the main valve member.
There is also a bleed opening in the housing or diaphragm, or through another channel, through which fluid can flow between a reservoir chamber above the diaphragm and an inlet chamber below the diaphragm. This bleed opening is smaller than the pilot opening. When the pilot opening is sealed by the plunger, fluid from the inlet port enters the inlet chamber below the diaphragm and passes through the bleed opening in the diaphragm to the reservoir above the diaphragm. The fluid above the diaphragm urges the diaphragm downwardly toward the main valve seat thereby sealing a main valve opening surrounded by the main valve seat, and closing the valve. When the solenoid is actuated to lift the plunger off of the pilot opening, fluid above the diaphragm is drained through the pilot opening faster than it can enter through the smaller bleed opening thereby lessening the pressure above the diaphragm and causing fluid pressure from the inlet below the diaphragm to force the diaphragm upward thereby lifting the main valve member off of the main valve seat for opening the valve.
The valve of the above mentioned U.S. Pat. No. 5,676,342 has been found to admirably perform its function. However when very low flow rates are to be maintained, the plunger is moved to a position which enables the diaphragm to lift the main valve member just slightly off of the main valve opening. At this time, the pressure differential between the areas above and below the diaphragm is so great that the main valve member tends to jump when lifted off of the main valve seat thereby preventing attainment of very low flow rates. This occurrence denotes the bottom end of the flow vs. current characteristic. That is, in a valve where flow rate is uniformly diminished by decreasing the current applied to the solenoid coil, flow is abruptly shut off when the solenoid coil current is reduced to a level where the main valve member is forced onto the main valve seat.
Conversely, while the main valve member is in engagement with the main valve seat and the current induced in the coil of a proportional solenoid valve is gradually increased, a level is reached where the main valve member jumps off of the main valve seat to a position where the lowest possible flow rate for that valve is achieved. Although this minimum flow rate can be optimized through careful selection of design parameters for the valve""s components, it can not be improved sufficiently in cases where precise low flow rates are required.
It is also known in the art to operate a solenoid valve at a constant high flow rate by applying to the valve solenoid a full wave AC current for displacing the main valve member from the main valve seat, and at a constant low flow rate by rectifying the AC current to obtain a half-wave AC signal which, when applied to the solenoid coil, enables fluid to pass through the pilot opening but does not provide sufficient lifting force to enable the main valve member to be lifted off of the main valve seat. Such a valve is the subject of U.S. Pat. No. 4,503,887 to Johnson et al.
It is further known in the art to vary the degree of displacement of a pilot valve member from a pilot valve seat in a proportional valve by applying power to the valve""s solenoid coil in the form of a periodically pulsed DC current, the amount of current varying. with the length of xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d times of the pulses, sometimes referred to as pulse width-modulation. Pulse width modulation for this purpose is disclosed in U.S. Pat. No. 5,294,089 to LaMarca and U.S. Pat. No. 5,676,342 to Otto et al.
None of the foregoing approaches has provided a solution to the problem of making a proportional solenoid valve with a high turn-down ratio, i.e., one which enables continuous variation of flow rate from very high and intermediate levels during which the main valve member is displaced from the main valve seat, to low levels during which the main valve member remains seated for sealing the main valve opening, and fluid flow is limited to passage through the pilot opening.
According to the invention, low flow rates are achieved over a continuous range, without lifting the main valve member off of the main valve seat, through pulse width and or frequency modulation of the current applied to the coil of a proportional solenoid valve. For low flow rates, e.g., gas flowing at a rate of 0.5 standard cubic feet per minute (scfm) to 5.0 scfm, the solenoid armature or plunger is oscillated or dithered onto and off of the pilot valve seat on the main valve member with a duty cycle during which the pilot opening is exposed to inlet fluid under pressure for a portion of the cycle, and the pilot opening is closed for the balance of the cycle thereby maintaining the main valve member on the main valve seat and limiting fluid flow to a path through the pilot opening. For increasingly greater flow rates, the duty cycle of the solenoid armature is adjusted to increase the proportion of the cycle during which the pilot opening is exposed to the fluid, and thereby increase the rate of fluid flow through the pilot opening.
As the rate of fluid flow approaches a level that can allow control of the displacement of the main valve member from the main valve seat without the problem of jumping which is encountered at lower flow rates, the duty cycle of the solenoid current is further adjusted to enable the pilot valve to remain open long enough to raise the main valve member from the main valve seat a distance corresponding to a desired intermediate rate of flow where the rate of flow through the pilot opening is supplemented by limited flow through the main valve opening. Flow at intermediate mass flow rates is permitted as the main valve member is lifted to a position a short distance from the main valve seat. Higher flow rates, to which the contribution of flow through the pilot opening becomes insignificant, are achieved as the main valve member is lifted further away from the main valve seat.
It is therefore an object of the invention to provide a single proportional flow valve, which can provide continuous variation of flow rates over a range heretofore unrealizable.
Another object of the invention is to provide a proportional flow valve with a solenoid actuator which can be energized by a current having a variable duty cycle for dithering a pilot valve member onto and off of a pilot seat on a main valve member for enabling a continuous range of low flow rates through a pilot opening in the valve without raising the main valve member from the main valve seat.
Still another object of this invention is to provide apparatus for modulating flow through the pilot opening in the seated main valve member without reaching the critical flow rate at which open the main valve member is lifted of off the main valve seat.
A further object of the invention is to provide a valve of the type described above wherein the duty cycle and/or frequency of the pulse width modulated solenoid current can be adjusted to enable the pilot valve to remain open long enough to raise the main valve member from the main valve seat in degrees corresponding to a desired rate of intermediate or high volume fluid flow.
Still another object of the invention is to maintain continuity between low flow, intermediate flow, and high flow rates in a proportional solenoid valve as a transition takes place from a range of low flow rates only through the pilot opening (main valve closed) through intermediate flow rates having significant components passing through both the pilot and main valve openings, to high flow rates which occur principally through the main valve opening.
Other and further objects of the invention will be apparent from the following drawings and description of a preferred embodiment of the invention in which like reference numerals are used to indicate like parts in the various views.